The invention relates to a navigation instrument for the navigation of land vehicles within a grid coordinate system by means of a heading reference unit.
It is known in sea navigation to determine the position of the ship from heading and speed. The speed is determined by means of a suitable log. The position is determined graphically on the nautical map. This kind of navigation is rather inaccurate as the compass measures only the direction between the longitudinal ship axis and the north direction, and the log measures only the speed of the ship through the water so that with this navigation procedure drift or leeway are not taken into consideration.
It is the object of the invention to provide a navigation instrument for the navigation of land vehicle which allows navigation without referring to landmarks such as roads, buildings and the like, and which indicates the position of the land vehicle very accurately. For this purpose, especially for military applications, the navigation is quite often not performed in a coordinate system based on geographical longitude and latitude but within an arbitrary grid coordinate system. Moreover, the position of this grid coordinate system in relation to the geographical coordinate system of longitude and latitude can, similar to a code, be established arbitrarily in order to conceal from the enemy the positions indicated within this grid coordinate system.
To achieve this object, a navigation instrument is provided in accordance with the invention characterized by
(a) a north seeking meridian gyro to determine the north direction, PA1 (b) a free gyro initially oriented with respect to the meridian gyro and used as a heading reference unit, PA1 (c) a movement pick-up for generating a signal representing the movement of the vehicle, PA1 (d) a computer connected to the heading reference unit and the movement pick-up, and arranged to generate output signals indicative of the vehicle position within the grid coordinate system from the grid heading angle signals and the movement signals provided by the heading reference unit and by the movement pick-up, and PA1 (e) means for compensating for the deviation of the free gyro relative to the grid coordinate system due to the earth's rotation by applying a follow-up signal to the gyro or by feeding this deviation into the computer. PA1 .phi. = geographical latitude PA1 R = radius of the earth PA1 V.sub.east = east component of the speed of the vehicle. PA1 (a) applying a corrective factor W.sub.A to the distance signal S.sub.gem measured as said movement signal or derived by integrating the speed to provide a corrected distance signal S on which the position computation is based, PA1 (b) computing the coordinates x.sub.F, Y.sub.F representing the vehicle position in the grid coordinate system as output signals of the computer from the grid heading angle signal .alpha..sub.K and the corrected distance signal S, PA1 (c) reading in of the coordinates x.sub.S and Y.sub.S of a known geodetic point whose position is established in the grid coordinate system, PA1 (d) forming the differences .DELTA.x, .DELTA.Y of the coordinates x.sub.F, Y.sub.F representing the vehicle coordinates and of the associated read-in coordinates x.sub.S, Y.sub.S of the geodetic point, PA1 (e) determining the corrections .DELTA..alpha..sub.K and .DELTA.W.sub.A or .DELTA.K.sub.v, respectively, of the grid heading angle signal and the corrective factor W.sub.A, respectively, of the distance signal S.sub.gem in accordance with the equations: ##EQU1## wherein S.sub.x and S.sub.y are the coordinate differences between the start point of the vehicle and the said known geodetic point, PA1 (f) correcting the output signals x.sub.F, Y.sub.F by the said differences .DELTA.x and .DELTA.y, respectively, and PA1 (g) continuing to compute the coordinates representing the vehicle position in the grid coordinate system with the corrective factor W.sub.A corrected by the correction .DELTA.W.sub.A and with the grid heading angle corrected by the correction .DELTA..alpha..sub.K. PA1 1. It corrects the output signals, so that the indication is brought into agreement with the coordinates of the true location, namely that of the geodetic point. PA1 2. For future position computation it corrects the grid heading angle signal .alpha..sub.K and the corrective factor W.sub.A to be applied to the movement signal s.sub.gem in accordance with the past error to correct, for example, for the slip.
The north seeking meridian gyro allows an accurate determination of the north direction prior to starting on a mission. The free gyro as a heading reference unit is oriented after the north direction thus determined. During the mission the computer then continuously determines the position within the grid coordinate system from the grid heading angles and the movement signals. Since in a land vehicle (which excludes a boat) the angle between longitudinal axis of the vehicle and heading reference rather accurately represents the direction of movement of the land vehicle, the heading can be determined exactly by the meridian gyro and the free gyro, the movement of the vehicle over land can also be measured rather accurately and, eventually, the evaluation is effected by means of a computer, the method of the invention -- other than similar methods in sea and air navigation -- permit a very accurate determination of the position.
The movement pick-up may be a speed pick-up or a distance pick-up.
A free gyro located in a vehicle tends to maintain its direction in space. This has the result that even if the gyro has its spin axis initially aligned exactly towards north, it slowly drifts from the north direction at an angle speed of EQU .omega..sub.Z = .omega..sub.E sin.phi. + (V.sub.east /R ) tg.phi. (1)
where
.omega..sub.E = speed of the rotation of the earth
The gyro spin axis must be slaved continuously with this angular speed if it has to be kept in north direction. The first term on the right side of equation (1) results from the fact that the north direction parallel to the surface of the earth in the meridian plane varies in space with the rotation of the earth so that gyro spin axis, which is stable in space without slaving, drifts with respect to the north direction. Such a drift of the gyro spin axis also occurs with respect to the grid coordinate system likewise rotating in space together with the earth.
The second term in equation (1) results from the fact that for a motion in east-west direction, the north direction (i.e., the direction of the tangent to the corresponding meridian) varies independently of the rotation of the earth. Such a change, however, does not occur with respect to the grid coordinate system not related to the earth, coordinate axes of which coincide only at the origin of the coordinate grid with the east-west and north-south directions, respectively.
If the globe were assumed to be immobile, a motion of the land vehicle with the gyro in east direction would not demand any change of the grid reference direction in space so that neither the angle formed by the direction stable in sapce of the gyro reference and the direction of the grid reference would be influenced by this.
In order to establish a fixed direction of the grid reference by means of the gyro, it is only necessary therefore to compensate for, or to take into account, the influence of the rotation of the earth on the gyro spin axis. For this purpose it is possible that the compensating means are arranged to generate a follow-up signal EQU .omega..sub.ZG = .omega..sub.E sin.phi. (2)
from the grid coordinates determined by the computer where, as indicated above, .omega..sub.E is the speed of rotation of the earth and .phi. is latitude.
It is, however, also possible that the computer is arranged to compute the deviation rate .omega..sub.ZG of the gyro from the determined grid coordinate and the geographical latitude which is derived therefrom, and to integrate that deviation rate with respect to time, taking into account the initial conditions, in order to determine the angle between the gyro reference direction and the grid reference direction, and in addition, the computer is arranged to take this angle into consideration during the generation of position output signals.
When the movement pick-up is a speed pick-up, the signal thereof is multiplied by the sine and cosine, respectively, of the respective grid heading angle and is integrated with respect to time, in order to determine the coordinates of the vehicle in the grid coordinate system. Due to an error .DELTA.K.sub.v of the scale factor of the speed pick-up there may be a distance error .DELTA.S. The error .DELTA.K.sub.v of the scale factor results in a speed error EQU .DELTA.V = .DELTA.K.sub.v .multidot. V.sub.gem,
wherein V.sub.gem is the speed of the vehicle as measured by the speed pick-up. By integration, a distance error is calculated of EQU .DELTA.S = .intg..DELTA.V dt = .DELTA.K.sub.v .intg. V.sub.gem dt EQU .DELTA.S = .DELTA.K.sub.v .multidot. S.sub.gem .
Electro-mechanical distance pick-ups are known which are either coupled directly to the drive wheels or are connected to a shaft of the gearbox of the vehicle. Thus such an electromechanical distance pick-up measures the number of revolutions of the drive wheels or of said shaft, respectively, and derives information therefrom as to the distance covered. The distance pick-up provides distance increment signals. The computer is arranged to multiply each distance increment signal, or a predetermined number of distance increment signals, by the sine and the cosine, respectively, of the grid heading angle and to sum up the products thus obtained, in order to derive the coordinates or the vehicle in the grid coordinate system.
Due to slip of the wheels or of the driving caterpillar track of the vehicle the information provided by the distance pick-up does not correspond exactly to the distance covered over ground. The relation between distance pick-up information and distance covered is not fixed and suitable for calibration but depends, for example, on the nature of the soil, the type of the topography and the weather conditions. Consequently, this may result in an error of the output signals from the computer indicating the position of the vehicle.
An equation for the distance error is valid which is formally identical with the above equation. A further source of error resides in a heading error .DELTA..alpha..sub.K by which the grid heading signal .alpha..sub.K provided by the heading reference unit is affected. Also such a heading error may result in considerable navigation errors.
It is a further object of the invention to improve the accuracy of navigation instruments of the type mentioned above.
According to the invention this object is achieved in that the computer carries out the following operations:
Thus with a navigation instrument designed in accordance with the invention, coordinates of a known geodetic point may be read into the computer. These coordinates are compared to the coordinates computed by the computer from heading and speed. On the basis of the difference between the true location and the computed location the computer carries out the following operation:
Thus after the vehicle has been driven from a start point to a geodetic point and at that point the corrections mentioned above have been applied to the computer, the further reading of the coordinates in the grid coordinate system will be effected with an accuracy that is considerably improved. It is essential in this connection that the coordinate errors .DELTA.x and .DELTA.y depend, as can be shown, only on the coordinate differences of the known start point and of said known geodetic point and do not depend on the path along which the vehicle proceeded from one point to the other.
"Geodetic point," in this connection, may be any landmark the coordinates of which are known very exactly. "Start point" may be the start point of the mission or a geodetic point in which the previous correction has been applied to the computer. For the computations commencing at the first "start point", the corrective factor W.sub.A to be applied to the measured distance may be read into the computer in accordance with empirical values taking the topographical and weather conditions into consideration.